I don't know the exact name of the files but you get the dual boot menu from the file boot.ini in root drive. There are other important ones with .sys and .bat extensions. Check through msconfig.
Have you given win98 a lot of space on your HD? If not, then simply set winXP as default and the timeout to zero in boot.ini but keep in mind that it won't remove win98, just sort of hide it and you would be able to see it through explorer or My computer.
If space is not a problem, then keep win98 on your pc since its installation only occupies something like 200-400 MB. It sometimes helps diagnose problems in your winXP installation.
See if that helps....